CHOLINERGIC BRADYCARDIA TEMPERATURE INDUCED IN ANTARCTIC FISHES NOTOTHENIA NEGLECTA AND CHAENOCEPHALUS ACERATUS*

Abstract: The cardiac responses of the Antarctic fishes Notothenia neglecta and Chaenocephalus aceratus to acute changes of temperature were studied. The fishes were surgically prepared for electrocardiogram (ECG) recording and placed in a three hour experiment in an experimental chamber containing aerated marine water. The temperature was controlled from +2'C to- 1.8 C. Presence of nitrites in the fish bath was monitored. Atropine and epinephrine were administered into the abdominal cavity through a surgically inserted cannula. Blood samples were taken at initial + 2. 0 T and final- 1. S T for cholinesterase assay. Results showed that: 1. A temperature decrease from +2.O0C to-- 1. 8 T significantly depressed cardiac rate, from 26.9k2.7 to 5.2+ 1.4 b e a t s m i n ' in N. neglecta and from 19.0 to 3.0 b e a t s m i n ' in C aceratus. In one specimen of N. neglecta. a reversible cardiac arrest of 3.0min was observed. ECG waves did not change during the experiments, except the QRS to QRS or the Q-T intervals at lower temperatures. Respiratory rate was not significantly influenced by temperature; 2. Return of temperature to + 1.5/2.0'C immediately ceased bradycardia; 3. Administration of atropine reverse